Power Converter with Power Switch Operable in Controlled Current Mode

ABSTRACT

A power converter and method of controlling a power switch therein to improve power conversion efficiency at low output current. In one embodiment, the power converter includes a first power switch coupled to a source of electrical power and a second power switch coupled to the first power switch and to an output terminal of the power converter. The power converter also includes a controller configured to alternately enable conduction of the first and the second power switches with a duty cycle in response to an output characteristic of the power converter. The controller is configured to control a level of current in the first power switch when the second power switch is substantially disabled to conduct.

TECHNICAL FIELD

The present invention is directed, in general, to power electronics and,more specifically, to a power converter and method of controlling apower switch therein to improve power conversion efficiency at lowoutput current.

BACKGROUND

A switch-mode power converter (also referred to as a “power converter”or “regulator”) is a power supply or power processing circuit thatconverts an input voltage waveform into a specified output voltagewaveform. DC-DC power converters convert a dc input voltage into a dcoutput voltage. Controllers associated with the power converters managean operation thereof by controlling the conduction periods of powerswitches employed therein. Generally, the controllers are coupledbetween an input and output of the power converter in a feedback loopconfiguration (also referred to as a “control loop” or “closed controlloop”).

Typically, the controller measures an output characteristic (e.g., anoutput voltage, an output current, or a combination of an output voltageand an output current) of the power converter, and based thereonmodifies a duty cycle of the power switches of the power converter. Theduty cycle is a ratio represented by a conduction period of a powerswitch to a switching period thereof. Thus, if a power switch conductsfor half of the switching period, the duty cycle for the power switchwould be 0.5 (or 50%). Additionally, as voltage or current for systems,such as a microprocessor powered by the power converter, dynamicallychange (e.g., as a computational load on the microprocessor changes),the controller should be configured to dynamically increase or decreasethe duty cycle of the power switches therein to maintain an outputcharacteristic such as an output voltage at a desired value.

In an exemplary application, the power converters have the capability toconvert an unregulated input voltage, such as five volts, supplied by aninput voltage source to a lower, regulated, output voltage, such as 2.5volts, to power a load. To provide the voltage conversion and regulationfunctions, the power converters include active power switches such asmetal-oxide semiconductor field-effect transistors (“MOSFETs”) that arecoupled to the voltage source and periodically switch a reactive circuitelement such as an inductor to the voltage source at a switchingfrequency that may be on the order of five megahertz.

In typical applications of dc-dc power converters, power conversionefficiency is an important parameter that directly affects the physicalsize of the end product, its cost and market acceptance. Active powerswitches that are either fully on with low forward voltage drop or fullyoff with minimal leakage current provide a recognized advantage forpower conversion efficiency in comparison with previous designs thatutilized a dissipative “pass” transistor to regulate an outputcharacteristic or a passive diode to provide a rectification function.Previous designs using pass transistors and passive diodes producedoperating power conversion efficiencies of roughly 40-70% in manyapplications. The use of active power switches in many recent powerconverter designs, particularly as synchronous rectifiers for low outputvoltages, has increased operating efficiency at full rated load to 90%or more.

A continuing problem with power converters is preserving powerconversion efficiency at low levels of output current. Low efficiency atlow output currents is a result of power inherently lost by parasiticelements in the power switches and by losses induced in reactive circuitelements, particularly inductors coupled to the active power switches.Further losses are also generated in the control and drive circuitscoupled to the active power switches. Ultimately, as the output currentof a power converter approaches zero, the fixed losses in the powerswitches, the inductive circuit elements, and the control circuits causepower conversion efficiency also to approach zero.

Various approaches are known to improve power conversion efficiency atlow output currents. One approach used with resonant power conversiontopologies reduces switching frequency of active power switches for lowoutput current. Another approach, as described by X. Zhou, et al., inthe paper entitled “Improved Light-Load Efficiency for SynchronousRectifier Voltage Regulation Module,” IEEE Transactions on PowerElectronics, Volume 15, Number 5, September 2000, pp. 826-834, which isincorporated herein by reference, utilizes duty cycle adjustments toadjust switching frequency or to disable a synchronous rectifier switch.A further approach, as described by M. E. Wilcox, et al. (“Wilcox”), inU.S. Pat. No. 6,580,258, entitled “Control Circuit and Method forMaintaining High Efficiency Over Broad Current Ranges in a SwitchingRegulator Circuit,” issued Jun. 17, 2003, which is incorporated hereinby reference, generates a control signal to intermittently turn off oneor more active power switches under light load operating conditions whenthe output voltage of the power converter can be maintained at aregulated voltage by the charge on an output capacitor. Of course, whenan output voltage from a power converter is temporarily discontinued,such as when the load coupled thereto is not performing an activefunction, the power converter can be disabled by an enable/disablesignal, generated either at a system or manual level, which is a processcommonly used, even in quite early power converter designs.

However, resonant power conversion topologies are frequently a poorchoice in many applications due to an inherently disadvantageouswaveform structure in resonant circuits and the resulting inefficientuse of semiconductor power switches required to execute the resonantpower conversion process at high levels of load current. Intermittentlyturning off one or more active power switches under light load operatingconditions as described by Wilcox still generates associated switchinglosses when the active power switches are periodically operated tomaintain charge on an output filter capacitor. Thus, the problem ofproviding high power conversion efficiency at light load currents stillremains an unresolved issue.

Accordingly, what is needed in the art is a power converter and relatedmethod to provide high power conversion efficiency in a switch-modepower converter, especially at light load currents, that overcomesdeficiencies in the prior art.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by advantageous embodimentsof the present invention, including a power converter and method ofcontrolling a power switch therein to improve power conversionefficiency at low output current. In one embodiment, the power converterincludes a first power switch coupled to a source of electrical powerand a second power switch coupled to the first power switch and to anoutput terminal of the power converter. The power converter alsoincludes a controller configured to alternately enable conduction of thefirst and the second power switches with a duty cycle in response to anoutput characteristic of the power converter. The controller isconfigured to control a level of current in the first power switch whenthe second power switch is substantially disabled to conduct.

In another aspect, the present invention provides a method of operatinga power converter including coupling a first power switch to a source ofelectrical power and coupling a second power switch to the first powerswitch and to an output terminal of the power converter. The method alsoincludes alternately enabling conduction of the first and the secondpower switches with a duty cycle in response to an output characteristicof the power converter. The method further includes controlling a levelof current in the first power switch when the second power switch issubstantially disabled to conduct.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a diagram of an embodiment of a power converterconstructed according to the principles of the present invention;

FIG. 2 illustrates a diagram of portions of the power converterillustrated in FIG. 1 constructed according to the principles of thepresent invention;

FIG. 3 illustrates a waveform diagram of an exemplary operationassociated with a power switch of a power converter in accordance withthe principles of the present invention;

FIG. 4 illustrates a diagram of an embodiment of portions of a powerconverter constructed according to the principles of the presentinvention;

FIGS. 5A to 5D illustrate waveform diagrams of exemplary operations of apower converter in accordance with the principles of the presentinvention; and

FIGS. 6A and 6B illustrate waveform diagrams of exemplary operationsassociated with a power switch of a power converter in accordance withthe principles of the present invention.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated, and may not beredescribed in the interest of brevity after the first instance. TheFIGUREs are drawn to clearly illustrate the relevant aspects ofexemplary embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently exemplary embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to exemplaryembodiments in a specific context, namely, a power converter including acontroller responsive to a level of output current to regulate an outputcharacteristic and methods of forming the same. While the principles ofthe present invention will be described in the environment of a powerconverter, any application that may benefit from a power converter, suchas a power amplifier, including a controller responsive to a level ofcurrent to regulate an output characteristic is well within the broadscope of the present invention.

Referring initially to FIG. 1, illustrated is a diagram of an embodimentof a power converter constructed according to the principles of thepresent invention. The power converter includes a power train 110, acontroller 120, and a driver (e.g., a gate driver) 130, and providespower to a system/load such as a microprocessor (not shown) coupled tooutput terminals 140, 141. The controller 120 is responsive to a levelof output current to regulate an output characteristic of the powerconverter. While in the illustrated embodiment the power train 110employs a buck converter topology, those skilled in the art shouldunderstand that other converter topologies such as a forward convertertopology are well within the broad scope of the present invention.

The power train 110 of the power converter receives an input voltageV_(in) from a source of electrical power (represented by battery 150) atan input thereof and provides a regulated output voltage V_(out) at theoutput terminals 140, 141, or other output characteristic such as anoutput current I_(out). In keeping with the principles of a buckconverter topology, the output voltage V_(out) is generally less thanthe input voltage V_(in) such that a switching operation of the powerconverter can regulate the output voltage V_(out).

In a first mode of operation wherein substantial output current I_(out)is delivered to the output terminals 140, 141, a main power switchQ_(mn) [e.g., a p-channel metal oxide semiconductor field effecttransistor (“MOSFET”) embodied in a p-type laterally diffused metaloxide semiconductor (“P-LDMOS”) device], is enabled to conduct inresponse to a gate drive signal S_(DRV1) for a primary interval(generally co-existent with a primary duty cycle “D” of the main powerswitch Q_(mn)) and couples the input voltage V_(in) to an output filterinductor L_(out). During the primary interval, an inductor currentI_(Lout) flowing through the output filter inductor L_(out) increases ascurrent flows from the input to the output of the power train 110. An accomponent of the inductor current I_(Lout) is filtered by an outputcapacitor C_(out) to provide the output current I_(out) at an output ofthe power converter. The power converter generally operates in the firstmode of operation, for example and without limitation, when the outputcurrent I_(out) is of sufficient magnitude that current in the outputfilter inductor L_(out) or in a power switch does not reverse direction.

During a complementary interval during the first mode of operation(generally co-existent with a complementary duty cycle “1-D” of the mainpower switch Q_(mn)), the main power switch Q_(mn) is transitioned to anon-conducting state and an auxiliary power switch Q_(aux) [e.g., ann-channel MOSFET embodied in an n-type laterally diffused metal oxidesemiconductor (“N-LDMOS”) device], coupled to the output filter inductorL_(out), is enabled to conduct in response to a gate drive signalS_(DRV2). The auxiliary power switch Q_(aux) provides a path to maintaina continuity of the inductor current I_(Lout) flowing through the outputfilter inductor L_(out). During the complementary interval, the inductorcurrent I_(Lout) flowing through the output filter inductor L_(out)decreases. In general, during the first mode of operation, the dutycycle of the main and auxiliary power switches Q_(mn), Q_(aux) may beadjusted to maintain a regulation of the output voltage V_(out) of thepower converter. Those skilled in the art should understand, however,that the conduction periods for the main and auxiliary power switchesQ_(mn), Q_(aux) may be separated by a small time interval to avoid crossconduction therebetween and beneficially to reduce the switching lossesassociated with the power converter. Those skilled in the art shouldunderstand further that terms used herein such as “current reversal” ora reference to a particular level of a physical quantity such as “zerocurrent” are to be understood within the context of a physical apparatuswith attendant and practical accuracy limitations. For example, onecannot know or measure the precise instant that a current that reversesdirection passes through a current level of zero.

The controller 120 of the power converter receives an outputcharacteristic (e.g., the output current I_(out) and/or the outputvoltage V_(out)) of the power converter, and a desired outputcharacteristic such as a desired system voltage V_(system) from aninternal source or from an external source that may be associated withthe load. In an advantageous embodiment, the controller 120 may becoupled to a current sensor, such as current sensor 160 to sense a powerconverter current such as an output filter inductor current I_(Lout) oran output current I_(out). In a further advantageous embodiment, thecontroller 120 may be coupled to a current sensor to sense a current ina power switch. Thus, a current sensor may be employed by controller 120to select the first mode of operation of the power converter bycomparing a sensed current with a fixed or adjustable current thresholdlevel.

The controller 120 may also be coupled to an input characteristic (e.g.,the input voltage V_(in)) of the power converter and to a return lead ofthe source of electrical power (again, represented by battery 150) asillustrated in FIG. 1 to provide a ground connection therefor. Whileonly a single ground connection is illustrated in the presentembodiment, those skilled in the art should understand that multipleground connections may be employed for use within the controller 120. Adecoupling capacitor C_(dec) may be coupled as illustrated in the FIGUREto the path from the input voltage V_(in) to the controller 120. Thedecoupling capacitor C_(dec) is generally configured to absorb highfrequency noise signals associated with the source of electrical powerto protect the controller 120.

In accordance with the aforementioned characteristics, during the firstmode of operation the controller 120 provides a signal (e.g., apulse-width modulated signal S_(PWM)) to control a duty cycle and afrequency of the main and auxiliary power switches Q_(mn), Q_(aux) ofthe power train 110 to regulate the output voltage V_(out) or otheroutput characteristic thereof. The controller 120 in some applicationsmay also provide a complement of the pulse-width modulated signalS_(PWM) during the first mode of operation (e.g., a complementarypulse-width modulated signal S_(1-PWM)) in accordance with theaforementioned characteristics. Any controller adapted to control atleast one power switch of the power converter is well within the broadscope of the present invention. As an example, a controller employingdigital circuitry is disclosed in U.S. Pat. No. 7,038,438, entitled“Controller for a Power Converter and a Method of Controlling a SwitchThereof,” to Dwarakanath, et al., issued May 2, 2006, and U.S. Pat. No.7,019,505, entitled “Digital Controller for a Power Converter EmployingSelectable Phases of a Clock Signal,” to Dwarakanath, et al, issued Mar.28, 2006, which are incorporated herein by reference.

The power converter also includes a driver (e.g., a gate driver) 130 toprovide gate drive signals S_(DRV1), S_(DRV2) to control conductivity ofthe main and auxiliary power switches Q_(mn), Q_(aux), respectively,responsive to the pulse-width modulated signal S_(PWM) (and, ifnecessary, the control the complementary pulse-width modulated signalS_(1-PWM)) provided by the controller 120. There are a number of viablealternatives to implement a driver 130 that include techniques toprovide sufficient signal delays to prevent crosscurrents whencontrolling multiple power switches in the power converter. The driver130 typically includes switching circuitry incorporating a plurality ofdriver switches that cooperate to provide the drive signals S_(DRV1),S_(DRV2) to the main and auxiliary power switches Q_(mn), Q_(aux). Ofcourse, any driver 130 capable of providing the drive signals S_(DRV1),S_(DRV2) to control a power switch is well within the broad scope of thepresent invention. As an example, a driver is disclosed in U.S. Pat. No.7,330,017, to Dwarakanath, et al., entitled “Driver for a PowerConverter and a Method of Driving a Switch Thereof,” issued Feb. 12,2008, and a power switch is disclosed in U.S. Pat. No. 7,230,302,entitled “Laterally Diffused Metal Oxide Semiconductor Device and Methodof Forming the Same,” to Lotfi, et al., issued Jun. 12, 2007 and in U.S.Pat. No. 7,214,985, entitled “Integrated Circuit Incorporating HigherVoltage Devices and Low Voltage Devices Therein,” to Lotfi, et al.,issued May 8, 2007, which are incorporated herein by reference.

According to the principles of the present invention, the main andauxiliary power switches Q_(mn), Q_(aux) are typically power switchesthat can be incorporated into a semiconductor device in an integratedcircuit proximate control or signal processing devices that perform manyof the control functions of the controller 120 of the power converter.The control and signal processing devices are typically complementarymetal-oxide semiconductor (“CMOS”) devices such as p-type metal oxidesemiconductor (“PMOS”) devices and n-type metal oxide semiconductor(“NMOS”) devices. The PMOS and NMOS devices may also be referred to asp-channel and n-channel MOSFETs, respectively.

In a switch-mode power converter, such as the buck power converterillustrated and described with reference to FIG. 1, the duty cycle of apower switch, such as the main power switch Q_(mn) previously describedherein, determines the steady-state ratio of a power converter outputvoltage V_(out) to its input voltage V_(in). In particular, for a buckpower converter typology operating in a continuous conduction mode, theduty cycle determines the ratio of output voltage to input voltage(ignoring certain losses within the power converter) according to theequation:

D=V _(out) /V _(in).   (1)

In an alternative power converter typology, such as a boost topology,the duty cycle determines the ratio of output to input voltage (again,ignoring certain losses within the power converter) operating in acontinuous conduction mode (“CCM”) according to the equation:

D=V _(in) /V _(out).   (2)

This reciprocal relationship for the ratio of input and output voltagesof buck and boost topologies recognizes that a buck power convertertopology employing synchronous rectifiers is operable as a boosttopology with its input and output reversed, and vice versa. Otherswitch-mode power converter topologies such as a buck-boost, forward,Cúk, etc., are characterized by further relationships, well known in theart, for a ratio of output voltage to input voltage, for a particularoperating condition such as CCM.

A controller, such as controller 120 illustrated in FIG. 1, typicallyregulates an output characteristic of a power converter by controlling aduty cycle of a power switch. The duty cycle is generally controlled bycomparing a sawtooth voltage waveform with a controlled thresholdvoltage produced by an error amplifier configured to sense an outputvoltage or other output characteristic.

A load coupled to a power converter may sometimes operate for a periodof time in an idle mode wherein the load draws a relatively small butnon-zero current from the power converter, for example, less than 1% ofits normal load current. Under such operating conditions, wherein powerconversion efficiency of the power converter is typically very low, itis preferable to provide high power conversion efficiency, particularlywhen the power converter is powered from a portable energy source suchas a battery.

Referring to FIG. 2, illustrated is a diagram of portions of the powerconverter illustrated in FIG. 1 constructed according to the principlesof the present invention. A switch-current controller 200 of the powerconverter is configured to control a current flowing through a mainpower switch Q_(mn) during the complementary duty cycle 1-D of aswitching cycle. The switch-current controller 200 includes a gatedriver 201 for the main power switch Q_(mn). The gate driver 201includes driver switches such as a p-channel field-effect transistor(“FET”) 224 and an n-channel FET 222, with their gates coupled togetherand driven by a pulse-width modulated signal S_(PWM). The pulse-widthmodulated signal S_(PWM) may be created by a controller such as thecontroller 120 illustrated and described with respect to FIG. 1. Thesource, gate, and drain of the respective driver switches are labeledwith “s,” “g,” and “d,” respectively.

Producing a current flowing through the main power switch Q_(mn) duringthe complementary duty cycle 1-D of a switching cycle when a powerconverter is lightly loaded, such as producing less than 5% of its ratedoutput power, can provide substantial efficiency improvement over aswitching mode wherein both the main and auxiliary power switchesQ_(mn), Q_(aux) are continuously enabled to conduct in complementaryintervals of time. A switching mode wherein the main and auxiliary powerswitches Q_(mn), Q_(aux) are enabled to conduct in complementaryintervals of time can produce substantial bidirectional current throughreactive circuit elements, such as an output inductor L_(out),contributing thereby substantial power losses. In addition, producing alight current flowing through the main power switch Q_(mn) during thecomplementary duty cycle 1-D of a switching cycle when a power converteris lightly loaded can be a more economical alternative than providing alarge output capacitance to maintain an output voltage V_(out) of apower converter during a mode of operation wherein all switching istemporarily disabled.

The switch-current controller 200 illustrated in FIG. 2 is configured tocontrol a gate voltage for the main power switch Q_(mn) when it wouldordinarily be disabled to conduct (i.e., during the complementary dutycycle 1-D of a switching cycle). The gate voltage of the main powerswitch Q_(mn) is controlled during the complementary duty cycle 1-D of aswitching cycle so that the main power switch Q_(mn) can conduct a lightcurrent to the load (i.e., an output current I_(out) of the powerconverter).

When the pulse-width modulated signal S_(PWM) is high, the gate driver201 couples the gate of the main power switch Q_(mn) to ground, turningit on. When pulse-width modulated signal S_(PWM) is low, the gate driver201 couples the gate of the main power switch Q_(mn) to the output of anoperational amplifier 214. The output of the operational amplifier 214is controlled to enable the main power switch Q_(mn) to conduct acontrolled, light current when the pulse-width modulated signal S_(PWM)is low.

A p-channel FET 212 with its gate coupled to its drain operates as adiode in forward conduction. The drain current of the p-channel FET 212is controlled by a resistor 216, which is coupled substantially acrossthe input voltage V_(in) (less the diode drop of the p-channel FET 212).Thus, the gate of the p-channel FET 212 is set to the voltage withrespect to its source that is necessary to conduct the current flowingthrough the resistor 216. This gate voltage is sensed with theoperational amplifier 214 and coupled to the gate of the main powerswitch Q_(mn) during the complementary duty cycle 1-D of a switchingcycle. Thus, the p-channel FET 212 and the main power switch Q_(mn)operate as a current mirror during the complementary duty cycle 1-D of aswitching cycle, wherein a current controlled by the resistor 216 andscaled by a die geometric ratio of the main power switch Q_(mn) to thep-channel FET 212 flows through the main power switch Q_(mn).Preferably, the p-channel FET 212 and the main power switch Q_(mn) areproduced in a common manufacturing process and are configured to operateat substantially the same die temperature. Preferably, the p-channel FET212 is a downscaled replica of the main power switch Q_(mn). Theoperation of current mirrors is well known in the art, and will not bedescribed further in the interest of brevity.

Turning now to FIG. 3, illustrated is a waveform diagram of an exemplaryoperation associated with a power switch of a power converter inaccordance with the principles of the present invention. In particular,the waveform represents a source-to-drain current I_(Qmn) flowingthrough a power switch (e.g., the main power switch Q_(mn) of FIG. 2)during a first mode of operation wherein the output current is ofsufficient magnitude that current in an output inductor or in asemiconductor power switch does not reverse direction. During theprimary duty cycle D of the switching cycle, the source-to-drain currentI_(Qmn) flowing through the main power switch Q_(mn) increasessubstantially linearly due to the voltage applied across an outputinductor (e.g., the output inductor L_(out) of FIG. 2). During thecomplementary duty cycle 1-D of the switching cycle, the source-to-draincurrent I_(Qmn) maintains a controlled, substantially constant currentlevel I_(CC) flowing through the main power switch Q_(mn), that iscontrolled by a controller such as the switch-current controller 200 ofFIG. 2.

Turning now to FIG. 4, illustrated is a diagram of an embodiment ofportions of a power converter constructed according to the principles ofthe present invention. A controller 400 of the power converter isoperable in three modes. The controller 400 regulates an outputcharacteristic of the power converter, and is configured to controlcurrent through a main power switch Q_(mn) in response to a sensed orestimated power converter current. The controller 400 is advantageouslyoperable to provide a mode of operation wherein improved powerconversion efficiency is achieved at light load. Additionally, the powerconverter may experience an improvement in dynamic response because ofan additional bias available to feed an output thereof that in turnproduces a smaller decay of an output characteristic (e.g., an outputvoltage V_(out)) since the output voltage V_(out) is not supplied onlyby an output capacitor C_(out). The power converter includes an erroramplifier 402 that senses the output voltage V_(out) as an outputcharacteristic to provide a feedback signal.

The primary and complementary duty cycles D and 1-D of a switching cycleare established by a comparator 414 that produces a pulse-widthmodulated signal S_(PWM). The noninverting input of the comparator 414is coupled to an error amplifier signal V_(EA) of an inverter 418. Theinverting input of the comparator 414 is coupled to a sawtooth waveformsignal V_(sawtooth) that has a substantial positive voltage offset forthe waveform valleys.

In a first mode of operation wherein a substantial current is deliveredto a load (not shown) coupled to output terminals 440, 441, the main andauxiliary power switches Q_(mn), Q_(aux) are alternately enabled toconduct, respectively, during a primary duty cycle D and a complimentaryduty cycle 1-D. The primary duty cycle D and the complimentary dutycycle 1-D are controlled to regulate an output characteristic of thepower converter. During the first mode of operation, the load current isof sufficient magnitude so that current flowing through output inductorL_(out) does not reverse direction. During the complementary duty cycle1-D, the main power switch Q_(mn) conducts a current controlled by acurrent mirror including a gate driver 401, an operational amplifier404, a p-channel FET 408 and a resistor 416. The current mirror isoperable in a manner similar to that described with reference to FIG. 2.However, in the circuit illustrated in FIG. 4, the current through themain power switch Q_(mn) is controlled by current flowing through theresistor 416. The resistor 416 is coupled to the error amplifier signalV_(EA) of the inverter 418. The inverter 418 amplifies the output of theerror amplifier 402 with gain -k. Thus, as the voltage of the erroramplifier signal V_(EA) is reduced, the current flowing through theresistor 416 is also reduced. Correspondingly, as the voltage of theerror amplifier signal V_(EA) is reduced, the current flowing throughthe main power switch Q_(mn) during the complementary duty cycle 1-D ofthe switching cycle is also reduced.

The error amplifier 402 senses the output voltage V_(out). The erroramplifier 402 includes an operational amplifier 409 that includesfeedback networks 405, 406. The feedback networks 405, 406 include aparallel arrangement of a capacitor and a resistor. The error amplifier402 further includes input networks including resistors 410, 412. In apreferred embodiment, the values of components in feedback networks 405,406 are equal, and the values of the resistors 410, 412 in the inputnetworks are equal. The selection of component values for an erroramplifier to produce a stable response of a power converter in aparticular application is well known in the art, and will not bedescribed further in the interest of brevity. The error amplifier 402generates the error amplifier signal V_(EA) in response to the sensedoutput voltage V_(out) of the power converter and a desired outputvoltage represented by signal V_(system). Of course, differentarrangements of feedback and input networks to meet the needs of aparticular application including a voltage divider coupled across outputterminals 440, 441 are well within the broad scope of the invention.Thus, in the first mode of operation, the controller 400 enablesalternating conduction of the main and auxiliary power switches Q_(mn),Q_(aux) while enabling a controlled current flowing through the mainpower switch Q_(mn) during the complementary duty cycle 1-D. Thecontrolled current flowing through the main power switch Q_(mn) isresponsive to the error amplifier signal V_(EA) produced by the erroramplifier 402.

In a second mode of power converter operation, power converter outputcurrent i_(out) is insufficient to sustain unidirectional current flowin output inductor L_(out). Bidirectional current flow through inductorL_(out) is prevented in the power converter illustrated in FIG. 4 bysensing an inductor current i_(Lout) with a current sensor 460. Thesensed inductor current i_(Lout) is amplified with a transresistanceamplifier that includes operational amplifier 426 coupled to feedbackresistor 428. The gain of the transresistance amplifier is substantiallythe resistance of the resistor 428. The output of the transresistanceamplifier is coupled to an input of an AND gate 420.

The output of the comparator 414 is coupled to a signal inverter 422.The signal inverter 422 produces a high output signal during thecomplementary duty cycle 1-D of the switching cycle. The output of thesignal inverter 422 is coupled to the other input of the AND gate 420.Thus, the AND gate 420 produces a high gate signal for the auxiliarypower switch Q_(aux) to enable conduction therein during thecomplementary duty cycle 1-D of the switching cycle, when positivecurrent flows through the output inductor L_(out). The current mirrorpreviously described continues to provide a gate drive signal for themain power switch Q_(mn) to enable a current controlled by the erroramplifier 402 to flow therethrough during the complementary duty cycle1-D of the switching cycle. In this manner, during the second mode ofpower converter operation, a controlled current is enabled to flowthrough the main power switch Q_(mn) when the auxiliary power switchQ_(aux) is disabled to conduct during the complementary duty cycle 1-Dof the switching cycle. The controller, therefore, is configured tocontrol a level of current in the main power switch Q_(mn) when theauxiliary power switch Q_(aux) is substantially disabled to conduct.

In a third mode of power converter operation, the output current i_(out)is further reduced so that a source-to-drain current that flows throughthe main power switch Q_(mn) is controlled to a controlled current levelI_(CC) to maintain sufficient current to power the load. As in the firstand second modes of power converter operation, the error amplifier 402can be used to control current flowing to the output terminal 440,thereby regulating an output characteristic of the power converter.

Turning now to FIGS. 5A to 5D, illustrated are waveform diagrams ofexemplary operations of a power converter in accordance with theprinciples of the present invention. In the interest of maintainingcontinuity, the waveform diagrams will be described in part withreference to signals and components illustrated and described withrespect to FIG. 4. More particularly, a left portion of FIG. 5Aillustrates an error amplifier signal V_(EA) produced by error amplifier402 and a voltage sawtooth signal V_(sawtooth) produced by a sawtoothvoltage generator (not shown) in the first mode of power converteroperation. When the error amplifier signal V_(EA) is greater then thevoltage sawtooth signal V_(sawtooth) produced by the sawtooth voltagegenerator, the comparator 414 illustrated in FIG. 4 sets the primaryduty cycle D of the switching cycle. In the right portion of FIG. 5A isa graphical representation of the resulting current that flows throughoutput inductor L_(out). During the primary duty cycle D of theswitching cycle, the current flowing through the output inductor L_(out)increases, and during the complementary duty cycle 1-D, the currentdecreases. In the right portion of FIG. 5A, the load current issufficiently high so that current in output inductor L_(out) does notreverse.

Referring now to FIG. 5B, illustrated in the left portion of the FIGUREagain is a graphical representation of the error amplifier signal V_(EA)produced by error amplifier 402 and the voltage sawtooth signalV_(sawtooth) produced by a sawtooth voltage generator, again in thefirst mode of power converter operation. In the right portion of FIG.5B, the current flowing through output inductor L_(out) is againillustrated. In this case, the current flowing through output inductorL_(out) reaches zero at the end of the switching cycle, but does notreverse direction, thus preserving operation in the first mode ofoperation.

Referring now to FIG. 5C, illustrated in the left portion of the FIGUREis a graphical representation of the error amplifier signal V_(EA)produced by error amplifier 402 and the voltage sawtooth signalV_(sawtooth) produced by a sawtooth voltage generator in a second modeof power converter operation. In the right portion of FIG. 5C, thecurrent flowing through output inductor L_(out) is again illustrated. Inthis case, the current flowing through output inductor L_(out) wouldreach zero and reverse direction during a primary duty cycle D*. In thissecond mode of power converter operation, the output current isinsufficient to prevent current reversal in output inductor L_(out)unless accommodation is provided in the power converter. Accommodationis provided by the AND gate 420 illustrated in FIG. 4 that disablesconduction in the auxiliary power switch Q_(aux) when current flowingthrough inductor L_(out) reaches zero or is less than zero. Thus, acontrolled current level I_(CC) is supplied to output inductor L_(out)by controlling current flowing through the main power switch Q_(mn). Theresult is a substantial reduction of ripple current conducted to outputcapacitor C_(out), with attendant reduction in switching-induced lossesin associated circuit components.

Referring now to FIG. 5D, illustrated in the left portion of the FIGUREis a graphical representation of the error amplifier signal V_(EA)produced by the error amplifier 402 and the voltage sawtooth signalV_(sawtooth) produced by a sawtooth voltage generator in a third mode ofpower converter operation. In this mode, the output current has beenreduced even further so that output current can be sustained by thecontrolled current level I_(CC) flowing through the main power switchQ_(mn) without alternately enabling conduction through the auxiliarypower switch Q_(mn) and the main power switch Q_(aux). In this mode, areduced voltage level for the error amplifier signal V_(EA) produced bythe error amplifier 402 causes the error amplifier signal V_(EA) to liebelow even the valleys of the sawtooth waveform signal V_(sawtooth).When the error amplifier signal V_(EA) lies entirely below the sawtoothwaveform signal V_(sawtooth), the comparator 414 produces no duty cycle(e.g., disabling the duty cycle). As illustrated in the right portion ofFIG. 5D, the current that flows through output inductor L_(out) is thecontrolled current level I_(CC) flowing through the main power switchQ_(mn), controlled by error amplifier 402 by means of the current mirroras described previously. The result is an output current controlled bythe error amplifier 402 that flows to the load without active switchingof either the main power switch Q_(mn) or the auxiliary power switchQ_(aux), represented again by the primary duty cycle D*. Little to noripple current is produced, and switching losses are substantiallyeliminated. Power conversion efficiency in this mode of operation issubstantially improved and is determined by the ratio of output voltageto input voltage and remaining losses in the power converter elements.In this mode of operation, some controller elements can be selectivelydisabled to further reduce power losses.

If, in an alternative embodiment, the level of controlled current I_(CC)for the main power switch Q_(mn) is not controlled by the erroramplifier 402 in response to actual load current at light levels of loadcurrent (i.e., a preselected level of controlled current level I_(CC) ischosen), it is important that the preselected level of controlledcurrent for the main power switch Q_(mn) be less than the expected levelof load current when the load is in a low-current state. Otherwise,output voltage of the power converter can increase beyond a desiredvoltage level.

Turning now to FIGS. 6A and 6B, illustrated are waveform diagrams ofexemplary operations associated with a power switch of a power converterin accordance with the principles of the present invention. In theinterest of maintaining continuity, the waveform diagrams will bedescribed in part with reference to signals and components illustratedand described with respect to FIG. 4. FIG. 6A illustrates a graphicalrepresentation of a primary duty cycle D resulting from the erroramplifier signal V_(EA) produced by error amplifier 402. When the erroramplifier signal V_(EA) is less than a threshold level represented bythe point 601, no duty cycle is produced (e.g., disabling the dutycycle). When the error amplifier signal V_(EA) is above this thresholdlevel, primary duty cycle D increases linearly with the error amplifiersignal V_(EA) until it reaches 100%. In a preferred embodiment, theerror amplifier 402 is constructed to produce an output error amplifiersignal V_(EA) that can fall below the minimum voltage of the sawtoothwaveform signal V_(sawtooth) to provide a mode of operation wherein noduty cycle is produced (e.g., disabling the duty cycle).

Referring now to FIG. 6B, illustrated is a graphical representation ofthe controlled level of current I_(CC) that flows through the main powerswitch Q_(mn) during the complementary duty cycle 1-D of the switchingcycle as a function of the error amplifier signal V_(EA) produced byerror amplifier 402. In a preferred embodiment, above an error amplifiervoltage represented by the point 602, the controlled current levelI_(CC) attains a saturation level 603. In an alternative embodiment,above the point 602, the controlled current level I_(CC) is reduced forfurther increases in the error amplifier signal V_(EA) beyond the point602 produced by error amplifier 402 as represented by dashed line 604.The controlled current levels I_(CC) illustrated in FIG. 6B arerepresented by straight lines, but in an alternative embodiment, it iscontemplated that these lines may be implemented as nonlinear functionsof the error amplifier signal V_(EA) to produce further efficiencyenhancements associated with a particular application.

Thus, as illustrated and described with reference to the accompanyingdrawings, a controller for a switch-mode power converter operable in aplurality of modes is constructed in an advantageous embodiment. Thecontroller is preferably formed with a single error amplifier that iscoupled to power converter elements to disable power switch conductivityupon reversal or reduction of a power converter current, and to providea controlled current in a main power switch when an auxiliary powerswitch is disabled to conduct. It is contemplated within the broad scopeof the invention that an operational mode can be changed based upon apower converter current, sensed or estimated, falling below a thresholdlevel. It is further contemplated that a controller can be constructedwith a plurality of error amplifiers to control an output characteristicin a plurality of operational modes. It is further contemplated that thecontroller can be constructed to operate in a plurality of operationalmodes based on sensing or estimating a parameter that may be an indirectindicator of an output characteristic such as an output current.

Those skilled in the art should understand that the previously describedembodiments of a power converter and related methods of constructing thesame are submitted for illustrative purposes only. In addition, otherembodiments capable of producing a power converter employable with otherswitch-mode power converter topologies are well within the broad scopeof the present invention. While the power converter has been describedin the environment of a power converter including a controller tocontrol an output characteristic to power a load, the power converterincluding a controller may also be applied to other systems such as apower amplifier, a motor controller, and a system to control an actuatorin accordance with a stepper motor or other electromechanical device.

For a better understanding of power converters, see “Modern DC-to-DCSwitchmode Power Converter Circuits,” by Rudolph P. Severns and GordonBloom, Van Nostrand Reinhold Company, New York, N.Y. (1985) and“Principles of Power Electronics,” by J. G. Kassakian, M. F. Schlechtand G. C. Verghese, Addison-Wesley (1991). The aforementioned referencesare incorporated herein by reference in their entirety.

Also, although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, many of the processes discussed above can be implemented indifferent methodologies and replaced by other processes, or acombination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods, and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A power converter, comprising: a first power switch coupled to asource of electrical power; a second power switch coupled to said firstpower switch and to an output terminal of said power converter; and acontroller configured to alternately enable conduction of said first andsaid second power switches with a duty cycle in response to an outputcharacteristic of said power converter, said controller configured tocontrol a level of current in said first power switch when said secondpower switch is substantially disabled to conduct.
 2. The powerconverter as recited in claim 1 wherein said controller includes anerror amplifier coupled to said output terminal of said power converterto control said level of current in said first power switch.
 3. Thepower converter as recited in claim 1 wherein said controller isconfigured to provide another mode of operation for said power converterwherein said duty cycle is disabled and said controller controls saidlevel of current in said first power switch in response to said outputcharacteristic.
 4. The power converter as recited in claim 1 whereinsaid second power switch is disabled to conduct in response to a sensedcurrent.
 5. The power converter as recited in claim 1 wherein secondpower switch is disabled to conduct in response to a sensedsubstantially reversed direction of current through an output inductorof said power converter.
 6. The power converter as recited in claim 1wherein said controller includes a current mirror to control said levelof current in said first power switch.
 7. The power converter as recitedin claim 1 wherein said output characteristic of said power converter isan output voltage of said power converter.
 8. The power converter asrecited in claim 1 wherein said controller provides two modes ofoperation for said power converter, comprising: a first mode ofoperation wherein said first and second power switches are alternatelyenabled to conduct in substantially complementary portions of aswitching cycle; and a second mode of operation wherein said first powerswitch is enabled to conduct for a portion of a switching cycle of saidpower converter, said second power switch is enabled to conduct for apart of a complementary portion of said switching cycle, whereinconduction of said second power switch is disabled in response to asensed current of said power converter.
 9. The power converter asrecited in claim 8 wherein said controller provides a third mode ofoperation for said power converter including disabling said alternatingconduction of said first and second power switches and controlling saidlevel of current in said first power switch.
 10. The power converter asrecited in claim 9 wherein in said third mode of operation said level ofcurrent in said first power switch is controlled in response to saidoutput characteristic.
 11. A method of operating a power converter,comprising: coupling a first power switch to a source of electricalpower; coupling a second power switch to said first power switch and toan output terminal of said power converter; alternately enablingconduction of said first and said second power switches with a dutycycle in response to an output characteristic of said power converter;and controlling a level of current in said first power switch when saidsecond power switch is substantially disabled to conduct.
 12. The methodas recited in claim 11 wherein said controlling is performed inaccordance with an error amplifier coupled to said output terminal ofsaid power converter to control said level of current in said firstpower switch.
 13. The method as recited in claim 11 further comprisingproviding another mode of operation for said power converter whereinsaid duty cycle is disabled and said controlling controls said level ofcurrent in said first power switch in response to said outputcharacteristic.
 14. The method as recited in claim 11 wherein saidsecond power switch is disabled to conduct in response to a sensedcurrent.
 15. The method as recited in claim 11 wherein second powerswitch is disabled to conduct in response to a sensed substantiallyreversed direction of current through an output inductor of said powerconverter.
 16. The method as recited in claim 11 wherein saidcontrolling is performed in accordance with a current mirror to controlsaid level of current in said first power switch.
 17. The method asrecited in claim 11 wherein said output characteristic of said powerconverter is an output voltage of said power converter.
 18. The methodas recited in claim 11 wherein said method provides two modes ofoperation for said power converter, comprising: a first mode ofoperation wherein said first and second power switches are alternatelyenabled to conduct in substantially complementary portions of aswitching cycle; and a second mode of operation wherein said first powerswitch is enabled to conduct for a portion of a switching cycle of saidpower converter, and said second power switch is enabled to conduct fora part of a complementary portion of said switching cycle, whereinconduction of said second power switch is disabled in response to asensed current of said power converter.
 19. The method as recited inclaim 18 wherein said method provides a third mode of operation for saidpower converter including disabling said alternating conduction of saidfirst and second power switches and controlling said level of current insaid first power switch.
 20. The method as recited in claim 19 whereinin said third mode of operation said level of current in said firstpower switch is controlled in response to said output characteristic.